1. Field of the Invention
This invention relates to an objective lens driving device for driving an objective lens for condensing a light beam on a signal recording surface of an optical recording medium, and an optical head having such lens driving device.
2. Description of the Related Art
Among recording mediums for information signals, there is an optical recording medium for recording/reproducing the information signals using an optical head. This recording medium may be enumerated by an optical disc, a magneto-optical disc or a phase-change optical disc. For increasing the information recording density of the optical recording medium, it is effective to increase the numerical aperture NA of the objective lens of the optical head. By increasing the numerical aperture NA of the objective lens, the spot size of the light beam condensed on the signal recording surface of the optical recording medium can be decreased to improve the information recording density.
Meanwhile, an objective lens of the conventional optical head, used for collecting the light from a light source on a signal recording surface, includes a sole lens. That is, the objective lens used in the conventional optical head is a so-called single-lens type lens.
If, in this optical head, the numerical aperture NA of the objective lens is to be increased, the single-lens type lens needs to have an extremely large refractive power. If the refractive power of the single-lens type lens is to be increased, the radius of curvature of the reflective surface needs to be extremely small, while refractive surfaces need to be positioned extremely accurately. Thus, if the objective lens of the optical head is a single-lens type lens, the numerical aperture NA cannot be raised beyond approximately 0.6.
In an optical recording medium, there is usually provided on the signal recording layer a layer adapted for protecting the signal recording surface (termed herein a cover glass). The light beam from the optical head falls on the signal recording surface via this cover glass. If the cover glass thickness deviates from its prescribed value, spherical aberration is produced. If the spherical aberration of a higher order is disregarded, this spherical aberration is proportionate to the numerical aperture NA raised to the fourth power. Therefore, if the numerical aperture NA of the objective lens is increased, the tolerance value of the thickness error of the cover glass is decreased significantly. That is, if the numerical aperture NA of the objective lens is increased, there is raised an extremely severe demand on the thickness tolerance of the cover glass to render it difficult to manufacture the optical recording medium.
As an optical head for solving the problem inherent in the conventional optical head, there has been proposed in U.S. Pat. No. 5,712,842 or U.S. Pat. No. 5,764,613 an optical head in which an objective lens is constituted by two lenses 101, 102, as shown in FIG. 1.
In this optical head, the objective lens is constituted by a first lens 101 on which falls the light beam from the light source and a second lens 102 on which falls the light beam converged by the first lens 101. The light beam from the light source is condensed by the first lens 101 and by the second lens 102 in this order before being illuminated on the optical recording medium. When the light beam is condensed by the first and second lenses 101, 102 so as to be then illuminated on the optical recording medium, the first and second lenses 101, 102 are moved by a biaxial actuator in a direction perpendicular to the optical axis in order o effectuate tracking servo. In addition, the first and second lenses 101, 102 are moved by the biaxial actuator along the optical axis in order to effectuate focussing servo.
In this optical head, if the refractive power of each of the first and second lenses 101, 102 is small, the two-lens system, consisting of the combination of the two lenses 101, 102, can have a large value of the numerical aperture NA. This realizes the numerical aperture exceeding 0.6, which it has been difficult to achieve with the optical head having a single-lens type lens as an objective lens.
Also, in the optical head employing the two-lens system, employing the biaxial actuator to effectuate tracking servo and focussing servo as described above, it is also possible to have a voice coil motor mounted on the second lens 102, instead of using the biaxial actuator, and to cause the second lens 102 to be moved by this voice coil motor along the optical axis to suppress the spherical aberration.
That is, not only the two-lens type lens system, made up of the first and second lenses 101, 102, is moved by the biaxial actuator, but also the second lens 102 is moved along the optical axis in the manner of cancelling the spherical aberration brought about by changes in film thickness of the cover glass of the optical recording medium by the voice coil motor mounted on the second lens 102, in order to reduce the spherical aberration.
That is, if, with the optical head having the two-lens type lens system as an objective lens, the voice coil motor is mounted on the second lens 102 to render it possible to adjust the distance between the first lens 101 and the second lens 102, it is possible to overcome the problem that the spherical aberration is increased with increased numerical aperture of the objective lens.
If, for cancelling the spherical aberration, the second lens 102 is adapted to be movable with respect to the first lens 101, it is necessary to increase the attenuation factor of the voice coil motor adapted for causing movement of the second lens 102 facing the optical recording medium. The attenuation factor of the voice coil motor can be increased by, for example, charging the viscous fluid. However, since the viscosity of the viscous fluid in general has temperature dependency, the viscous fluid is changed in viscosity by fluctuations in the external air temperature if the viscous fluid is charged to increase the attenuation factor of the voice coil motor, with the result that the focussing servo is rendered unstable.
For maintaining stability of focussing servo, the attenuation factor of the voice coil motor mounted on the second lens 102 needs to be increased. However, if the attenuation factor of the voice coil motor is increased, there is produced hysteresis in displacement characteristics with respect to the input current to render it impossible to realize high-speed stable operation.
Also, with the two-lens type lens system shown in FIG. 1, the second lens 102 facing the optical recording medium can be displaced, so that, if the second objective lens 102 collides against the optical recording medium, the distance between the first lens 101 and the second lens 102 is varied significantly, with the result that it is difficult to maintain an appropriate distance between the two lenses. Moreover, if the second lens 102 collides against the optical recording medium, there is imposed a large load on the member movably supporting the second lens 102, such that there is the risk of destruction of the supporting member.
It is therefore an object of the present invention to provide a lens driving device for driving an objective lens designed to converge a light beam on an optical recording medium in which the numerical aperture NA of the objective lens can be increased to suppress the spherical aberration and in which focussing servo can be effectuated speedily and stably.
It is another object of the present invention to provide an optical head having such lens driving device.
In one aspect, the present invention provides a lens driving device for driving a lens adapted for collecting the light from a light source on a signal recording surface of an optical recording medium, first driving means having a movable portion which is adapted for being moved in a direction along the optical axis of the light from the light source and in a direction perpendicular to the optical axis, second driving means mounted on a movable portion of the first driving means and having a movable portion movable in the direction along the optical axis of the light from the light source, a first lens on which falls the light from the light source, and a second lens on which falls the light collected by the first lens. The second driving means is mounted on the movable portion of the fist driving means and the first lens is mounted on the movable portion of the second driving means.
In the present lens driving device, the light from the light source is collected by the first lens and by the second lens in this order so as to be illuminated on the signal recording surface of the optical recording medium. The second lens mounted on the movable portion of the first driving means, the second driving means and the first lens mounted on the movable portion of the second driving means are driven in the direction along the optical axis and in a direction perpendicular to the optical axis to effectuate tracking servo and focussing servo. In the present lens driving device, the first lens can be driven in the direction along the optical axis to cancel the spherical aberration.
If, in the present lens driving device, an amount of movement xcex94Z of the focal point position of the light collected by the first lens and the second lens when the first lens is moved along the optical axis by xcex94Z1 and the second lens is moved along the optical axis by xcex94Z2 is expressed with a coefficient xcex1 by the following equation
xcex94Z=(xcex94Z1+xcex1xc2x7xcex94Z2)(1+xcex1),
the ratio of a weight m2 of the movable portion of the first driving means and the member mounted on the movable portion to a weight m1 of the movable portion of the second driving means and the member mounted on the movable portion, or m2/m1, is preferably not larger than the coefficient xcex1. By setting the ratio m2/m1 so as to be not larger than xcex1, open loop characteristics in case of focussing servo are improved.
Also, in the present lens driving device, the second driving means is a voice coil motor which has its upper and lower ends along the optical axis supported by elastic supporting members, and which is adapted for driving the movable portion in a direction along the optical axis. The center of gravity of the movable portion and the member mounted on the movable portion in the direction along the optical axis is in the vicinity of a mid point along the optical axis of an elastic supporting member supporting the upper end of the movable portion and an elastic supporting member supporting the lower end of the movable portion.
In another aspect, the present invention provides an optical head including a light source, lens driving means for driving a lens adapted for collecting the light on a signal recording surface of an optical recording medium and light receiving means for receiving the return light which is collected on the signal recording surface of the optical recording medium by the lens driven by the lens driving means and which is reflected back by the optical recording medium. The the lens driving means includes first driving means having a movable portion which is adapted for being moved in a direction along the optical axis of the light from the light source and in a direction perpendicular to the optical axis, second driving means mounted on a movable portion of the first driving means and having a movable portion movable in the direction along the optical axis of the light from the light source, a first lens on which is incident the light from the light source and a second lens on which is incident the light collected by the first lens. The second driving means is mounted on the movable portion of the first driving means. The first lens is mounted on the movable portion of the second driving means, while the second lens is mounted on the movable portion of the first driving means.
In the present optical head, the light from the light source is collected by the first lens and by the second lens in this order so as to be illuminated on the signal recording surface of the optical recording medium. The second lens mounted on the movable portion of the first driving means, second driving means and the first lens mounted on the movable portion of the second driving means is driven in the direction along the optical axis and in a direction perpendicular to the optical axis to effectuate tracking servo and focussing servo. In the present optical head, the first lens can be driven in the direction along the optical axis to cancel the spherical aberration.
Also, if, in the present optical head, an amount of movement xcex94Z of the focal point position of the light collected by the first lens and the second lens when the first lens is moved along the optical axis by xcex94Z1 and the second lens is moved along the optical axis by xcex94Z2 is expressed with a coefficient xcex1 by the following equation
xcex94Z=(xcex94Z1+xcex1xc2x7xcex94Z2)(1+xcex1),
the ratio of a weight m2 of the movable portion of the first driving means and the member mounted on the movable portion to a weight m1 of the movable portion of the second driving means and the member mounted on the movable portion, or m2/m1, is preferably not larger than the coefficient xcex1. By setting the ratio m2/m1 so as to be not larger than xcex1, open loop characteristics in case of focussing servo are improved.
Moreover, in the present optical head, the second driving means is a voice coil motor having its upper and lower ends along the optical axis supported by elastic supporting members, and which is adapted for driving the movable portion in a direction along the optical axis. Preferably, the center of gravity of the movable portion and the member mounted on the movable portion in the direction along the optical axis is in the vicinity of a mid point along the optical axis of an elastic supporting member supporting the upper end of the movable portion and an elastic supporting member supporting the lower end of the movable portion.
According to the present invention, as described above, there is provided a lens driving device in which the numerical aperture NA of the objective lens can be enlarged and spherical aberration can be suppressed, while focussing servo can be realized speedily and in stability. In addition, there is provided an optical head having such lens driving device.